Proximity detectors for detecting ferromagnetic objects are known. In proximity detectors, the magnetic field associated with the ferromagnetic object is detected by a magnetic field sensing element, such as a Hall effect element or a magnetoresistance element, which provides a signal (i.e., a magnetic field signal or sensing element signal) proportional to a detected magnetic field.
Some types of magnetic field sensors, i.e. proximity detectors, merely provide an output signal representative of the proximity of the ferromagnetic object. However, other types of magnetic, field sensors, i.e., rotation detectors, provide an output signal representative of the approach and retreat of each tooth of a rotating ferromagnetic gear or of each segment of a segmented ring magnet having segments with alternating polarity as the gear or ring magnet rotates. The rotation detector processes the magnetic field signal to generate an output signal that changes state each time the magnetic field signal either reaches a value near to a peak (positive or negative peak) or crosses a threshold level. Therefore, the output signal, which has an edge rate or period, is indicative of a rotation and a speed of rotation of the ferromagnetic gear or of the ring magnet.
One type of rotation detector can compare a sinusoidal sensing element signal to a threshold. In some types of rotation detectors, a peak-to-peak percentage detector (or threshold detector) generates at least one threshold level that is equal to a percentage of the peak-to-peak magnetic field signal detected by one or more magnetic field sensing elements. For this type of rotation detector, the output signal changes state when the magnetic field signal crosses the at least one threshold level. One such threshold detector is described in U.S. Pat. No. 5,917,320 entitled “Detection of Passing Magnetic Articles While Periodically Adapting Detection Threshold” assigned to the assignee of the present invention and incorporated herein by reference.
In another type of rotation detector, a slope-activated detector, also referred to as a peak-referenced detector (or peak detector), threshold levels are identified that differ from the positive and negative peaks (i.e., the peaks and valleys) of the sensing element signal by a predetermined amount. Thus, in this type of rotation detector, the output signal changes state when the magnetic field signal departs from a peak and/or valley by the predetermined amount. One such peak detector is described in U.S. Pat. No. 6,091,239 entitled “Detection Of Passing Magnetic Articles With a Peak Referenced Threshold Detector,” which is assigned to the assignee of the present invention and incorporated herein by reference, Another such peak detector is described in U.S. Pat. No. 6,693,419, entitled “Proximity Detector,” which is assigned to the assignee of the present invention and incorporated herein by reference. Another such peak detector is described in U.S. Pat. No. 7,199,579, entitled “Proximity Detector,” which is assigned to the assignee of the present invention and incorporated herein by reference.
It should be understood that, because the above-described peak-to-peak percentage detector (threshold detector) and the above-described peak-referenced detector (peak detector) both have circuitry that can identify the positive and negative peaks of a magnetic field the peak-to-peak percentage detector and the peak-referenced detector both include a peak detector circuit configured to detect a positive peak and a negative peak of the magnetic field signal. Each, however, uses the detected peaks in different ways.
In order to accurately detect the positive and negative peaks of a magnetic field signal, some rotation detectors are capable of tracking at least part of the sensing element signal (magnetic field signal). To this end, typically, one or more digital-to-analog converters (PACs) can be used to generate a tracking signal, which tracks the magnetic field signal. For example, in the above-referenced U.S. Pat. Nos. 5,917,320 and 6,091,239, two DACs are used, one (PDAC) to detect the positive peaks of the magnetic field signal and the other (NDAC) to detect the negative peaks of the magnetic field signal.
As described above, an output signal generated by a conventional proximity detector used to detect a rotation of a ferromagnetic object (e.g., a ring magnet or a ferromagnetic gear) can have a format indicative of the rotation and of the speed of rotation of the ferromagnetic object or ring magnet. For example, the conventional proximity detector can generate the output signal as a two-state binary signal having a frequency indicative of the speed of rotation. In some arrangements, the output signal can be comprised of voltage or current pulses, a rate of which is representative of speed of rotation, and a puke width of which is indicative of direction of rotation. This arrangement is described, for example, in U.S. patent application No. 6,815,944, issued Nov. 9, 2004, assigned to the assignee of the present invention, and incorporated by reference herein in its entirety.
In conventional rotation detectors, the above-described pulses are generated at a rate that features on a ferromagnetic object pass by the proximity detector. A variety of types and shapes of ferromagnetic objects can be used.
In some arrangements, the ferromagnetic object is a gear like object having gear teeth and the magnetic field sensor, e.g., rotation detector, is a back-biased magnetic field sensor, which includes a magnet to generate a magnetic field proximate to the magnetic field sensor. Gear teeth passing by the magnetic field sensor cause changes in the strength and angle of the magnetic field, and thus, the passing gear teeth can be sensed and the above-described pulses can be generated with a rate at which the gear teeth pass by.
In other arrangements, the ferromagnetic object is a ring magnet having one or more north-south pole pairs. These arrangements do not need the back-biased arrangement and the north-south pole pairs passing by the proximity detector can be sensed and the above-described pulses can be generated with a rate at which the north-south pole pairs pass by the magnetic field sensor.
In both of the above arrangement, it should be apparent that there is no information provided by the magnetic field sensor, e.g. rotation detector, between the pulses. However, there are applications for which it is desirable that the magnetic field sensor provides a higher resolution of angular accuracy of the ferromagnetic object.
For example, an automobile can have an automatic parking assist function that can use the ABS (automatic braking system) components to detect a rotational speed of a wheel. The automatic parking assist operates at very low wheel speeds, and thus, it is desirable to identify a wheel rotation position with a higher resolution than is typically provided by the ABS system.